The present invention generally relates to methods for fabricating solid state image sensors and, more particularly, to a method for fabricating thinned, back-illuminated, solid state image sensors that have an inherently high quantum efficiency over a broad spectral range.
Thinned solid state image sensors, such as charge coupled devices (CCD's), are widely used in the field of image processing. These sensors are generally used to provide electrical signal representations of incident electromagnetic radiation intensities over a particular spectral band. Of course, the most common spectral band used for image processing is the visible band.
Thinned solid state image sensors, particularly CCD's, are typically made of silicon to take advantage of the properties of the silicon crystal lattice. In crystalline form, each atom of silicon is covalently bonded to its neighboring silicon atoms resulting in an energy band gap of approximately 1.12 eV. To break one of the covalent bonds, and thus create an electron-hole pair, an energy greater than the energy band gap is required. Incident electromagnetic radiation in the form of photons having wavelengths shorter than 1 .mu.m possess such energy.
The wavelength of an incoming photon (light), and the depth to which an incoming photon penetrates into a piece of silicon are directly related; the shorter the wavelength, the shorter the penetration depth. Silicon becomes transparent to light at a wavelength of approximately 1100 nm and is essentially opaque to light at wavelengths shorter than 400 nm. Thus, when designing solid state image sensors, the thickness of a silicon base layer, as well as many other parameters, can be chosen to optimize performance over a specific spectral band.
In order to measure the electronic charge produced by incident photons in, for example, a CCD, a means for collecting this charge is required. Thus, potential wells are formed, wherein a thin insulating layer of silicon dioxide is grown on a silicon base layer, and a conductive gate structure is formed over the silicon dioxide layer. The gate structure is formed in an array of columns and rows, thus making it possible, by applying a positive electrical potential to the various gate elements, to create depletion regions, or potential wells, in the silicon base layer where free electrons generated by the incoming photons can be stored.
By controlling the electrical potential applied to adjacent gates, depletion regions, or potential wells, containing free electrons can be caused to migrate along a column, or row, so that a signal may eventually be output at the edge of the array. Typically, the gate structure is arranged using multiple phases, usually three phases, so that the potential wells may be successively migrated through the silicon to the array edge. It should be noted, however, that since the charge in the potential wells located far from the array edge must undergo hundreds of transfers, the charge transfer efficiency of the CCD is a critical parameter, as is the quantum efficiency and the spectral response. These considerations are of particular importance when extremely low light levels are to be sensed.
In a front-illuminated solid state image sensor, such as the CCD described above, light normally enters the silicon base layer by passing through the gates formed on the silicon dioxide layer. The gates are usually fabricated of very thin polysilicon, which is reasonably transparent to long wavelengths but becomes opaque at wavelengths shorter than 400 nm. Thus, at short wavelengths, the gate structure greatly attenuates incoming light.
To overcome the attenuation problems of front-illuminated solid state image sensors, it has become common practice to uniformly thin the silicon base layer to a thickness on the order of 10 .mu.m using conventional chemical etching techniques. After thinning, it becomes possible to focus an image on the back silicon surface of a CCD, where there is no gate structure to attenuate the incoming light. Thus, a thinned, back-illuminated, solid state image sensor is formed having an improved quantum efficiency and spectral response. These devices have been found to exhibit high sensitivity to radiation from the soft X-ray to the near infrared regions of the electromagnetic spectrum.
Although the conventional chemical etching techniques used to thin the silicon base layer as described above are sufficient in their prescribed functions, they nevertheless result in several unwanted consequences. First, the chemical etchants used leave the back silicon surface roughened with surface variations on the order of 500 Angstroms and frequent etch pits. Thus, the resulting surface is severely wrinkled, and even if flattened by attaching to a support structure, significant non-planarity remains. Such non-planarity degrades performance, especially when used in fast (small f-number) optical systems. Secondly, the chemical etching techniques used require the active CCD circuitry on each pixel face to be protected during the etching process. Typically, each pixel face is waxed to a support substrate during the etching process and transferred to a second, optically transparent, substrate thereafter. This approach results in excessive handling of the CCD's, and thus significantly increases the possibility of damage thereto.
Although not previously addressed, another problem exists with the above-described method of fabricating thinned, back-illuminated, solid state image sensors. Due to the fact that the thinned silicon surface is left bare after the etching process, it is subject to surface charging, surface state formation, and other contamination problems all of which are detrimental to device performance. The fundamental problem is that no fully effective method has been found to passivate the back silicon surface after the thinning process is completed. Thermal oxidation of the back silicon surface, the established technique for high quality passivation of silicon surfaces, is not possible because the high temperatures involved (greater than 900 degrees Celsius) would destroy the active CCD circuitry. Accordingly, it is desirable to overcome this passivation problem, and the other above-mentioned shortcomings associated with fabricating thinned, back-illuminated solid state image sensors.